Google XML Sitemaps and Domain Mapping

I'm a newbie aggressively trying to solve an issue in regards to the Google XML Sitemaps file not referencing the domain we are mapped to.

Will the plugin Simple Sitemaps solve this problem or would adjusting settings in the Google XML sitemaps plugin do the trick? I see two areas in XML sitemaps "Additional pages" and "Location of your sitemap file", and I wonder if either is pertinent to my problem.

Instead of seeing this in the sitemap.xml:
http://mymappeddomain.com.main.wpmudomain.com/

I want to see this:
http://mymappeddomain.com

Also, is the xml file location relevant, as in I should see http://mymappeddomain.com/sitemap.xml instead of http://mymappeddomain.com.main.wpmudomain.com/sitemap.xml?

Hope this question makes sense. Any help appreciated.

Thank